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MICROLITHOGRAPHY

MICROLITHOGRAPHY 9.17

1.6

1.4

1.2
Relative intensity 0.8 1

0.6

0.4
0.2

0
0 200 400 600 800 1000
Depth into resist (nm)
FIGURE 9.11 Standing wave intensity in 1 µm of photoresist on a silicon substrate for a normally
incident plane wave of 365 nm wavelength.

for reducing the standing wave intensity that Eq. (9.13) suggests is to increase absorption in the resist
(reduce the e −aD term). This is accomplished by adding a dye to the photoresist (increasing a).

9.3 PHOTORESIST CHEMISTRY

The formation of an aerial image is only the first step in the transfer of information from a pho-
tomask into a resist pattern. The aerial image must propagate into the resist and cause a chemical
change, forming a latent image of exposed and unexposed material. This latent image, either direct-
ly or indirectly, will affect the solubility of the resist, allowing the latent image to be turned into a
profile image through the process of development.

9.3.1 Exposure Kinetics
All photoresists have a light sensitive compound called a sensitizer that reacts when exposed to light
of a certain wavelength. For conventional resists used at near UV wavelengths, this molecule is
called a PAC. The chemistry of exposure for diazonaphthoquinone (a popular PAC) is given below

O
N 2 C O COOH
UV H O
2
+ N 2
SO 2 SO 2 SO 2

R R R (9.14)

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